This invention relates to a device for measuring air-fuel ratio of combustion engines.
In various industrial fields, there have been used engines which generate combustion energy by burning air with alcoholic fuel such as methanol and ethanol, or hydrocarbon fuel such as gasoline, gas oil and propane. Among other applications, internal combustion engines have been widely used in automobiles.
For design, manufacture and maintenance of these internal combustion engines, it is important that the ratio of an amount of air to that of fuel consumed in an engine (the ratio is normally defined as mass ratio and is called air-fuel ratio hereinafter) is measured to increase engine efficiency and to prevent environmental pollution.
In conventional methods for measuring the air-fuel ratio, there have been used, in recent years, a residual oxygen density measuring method and a residual carbon dioxide density measuring method in both of which an amount of exhaust gas is sampled to measure the density of residual oxygen and carbon dioxide therein, respectively. In these methods, it is desirable that accurate air-fuel ratio can be obtained, these measuring principles are easily understandable, these measuring mechanisms are simple, the ratio can be measured for a short time and these measuring operations are convenient.
In an air-fuel ratio measuring device for carrying out the above methods, an amount of exhaust gas is sampled and completely oxidized through a catalyst member for oxidation and thereafter the density of residual oxygen is measured. Thus, the air-fuel ratio can be measured irrespective of the amount of sampled exhaust gas. Instead of the density of residual oxygen, the density of residual carbon dioxide may be measured to obtain the air-fuel ratio.
Functional expressions for obtaining the air-fuel ratio by the above residual oxygen density measuring methods are illustrated by the following expressions (1) to (3). The expression (3) must be slightly amended in the case of the residual dioxide carbon density measuring method.
Calculation Expressions: EQU AFR=.lambda..multidot.AFRstoic (1) ##EQU1## wherein, AFR: air-fuel ratio (mass ratio)
AFRstoic: stoichiometric air-fuel ratio PA1 .lambda.: equivalence ratio PA1 n: H/C ratio (number of hydrogen atoms per carbon atom in fuel) PA1 y: O/C ratio (number of oxygen atoms per carbon atom in fuel) PA1 C: density of residual oxygen
Residual oxygen always exists when air is supplied to a combustion engine in more than a theoretically necessary amount for complete combustion (this condition is called lean burn region). In this case, the measurement of the residual oxygen density is possible. However, residual oxygen does not exist when air is supplied to a combustion engine in a theoretically necessary amount for complete combustion (this is called theoretical ratio) or less than that (this condition is called rich burn region). In these cases, the measurement of the residual oxygen is impossible.
To solve the above problem in the rich burn region, an amount of exhaust gas to be sampled is diluted with a predetermined amount of air. A calculation expression in this case is as follows. ##EQU2## wherein Z is air dilution ratio.
Internal combustion engines are often operated in the rich burn region. Therefore, dilution mechanisms for diluting sampled exhaust gas with air are very important to measure accurately the air-fuel ratio.
There have been two typical air dilution mechanisms.
The first conventional air dilution mechanism has an exhaust gas suction line for sucking exhaust gas and an air suction line disposed in parallel with the exhaust gas suction line in order to suck air for dilution from a high pressure air source. The exhaust gas suction line and air suction line are connected to each other at a junction point from which a mixture line for transmitting a mixture fluid of the exhaust gas and the dilution is extended. On the exhaust gas suction line are provided a suction pump and a pressure adjustment valve with a servo-mechanism. On the air suction line are provided a motor drive valve and a pressure adjustment valve with a servo-mechanism, and on the mixture line are provided a catalyst member, an oxygen density sensor, a pressure adjustment valve with a servo-mechanism and an exhaust pump.
The above lines having an air dilution mechanism are provided with pressure adjustment valves with servo-mechanisms. Accordingly, the air-fuel ratio can be measured steadily even if the pressure of the exhaust gas and the dilution air changes in a wide region. However, the pressure adjustment valves are not only expensive but also complicated in construction.
In contrast, the second conventional air dilution mechanism has a plurality of capillary tubes on an exhaust gas suction line and an air suction line, respectively, and a mixture line is extended from a junction point of the two lines. The mixture line has a catalyst member, an oxygen density sensor and an exhaust pump. The capillary tubes are so designed that their Reynolds number is very low. The second mechanism is relatively inexpensive and easily maintained. However, the air dilution ratio cannot be consistently calculated if the pressure of the sampled exhaust gas changes and if the viscosity of the exhaust gas changes due to the change of its temperature.
Further, in combination engines, there often occur pulsations with a large amplitude and a short cycle. In such cases, even the first mechanism is not useful because the pressure adjustment valves with servo-mechanisms cannot overcome such pulsations.